Conventionally, cleaning of oil contaminated industrial waste water is performed by passing the waste water through separation ponds in which the oil rises to the surface and is removed partially by sweeping. This process may be combined with a flotation, in which air is dispersed into the water and promotes the separation of oil droplets and collection thereof at the water surface.
This treatment may be followed by passage of the partially cleaned water through a sandfilter or filter elements.
When it is necessary to remove not only emulsified or suspended contaminations but also such which are dissolved in the aqueous phase a subsequent filtration through a filter comprising activated carbon is conventional.
For purification of only moderately oil-contaminated water it has been suggested to filter through a porous polymer material, pre-impregnated with an absorption liquid. However, this method is not suited as the sole treatment of more heavily polluted water.
In e.g. U.S. Pat No. 5,591,347 (Cairo, Jr. et al.), an apparatus is disclosed in which a fluidization is performed in the upper portion of a vessel whereas a filtration takes place in the lower part of the same vessel.
Although the last-mentioned type of apparatuses represents certain improvements, especially reduced space requirements in comparison to equipments having distinct flotation and filtration devices, the purification efficiency is inferior to the one desired to fulfil the environmental standards of today.
A different concept is used in the filter apparatus disclosed in U.S. Pat. No. 4,839,040 (Alm). Said apparatus has a vertical annular filter chamber enclosing a particulate, elastic, oil absorbing, regeneratable filter material. The efficient use of such a filter material is enabled by axially moveable annular transverse walls for continously compressing the filter material. The vertical outer walls of the annular filter chamber are encircled by a jacket which receives the contaminated water through an inlet in the lower portion of the jacket. While present in said jacket during its passage towards the filter surface, a partial gravitational separation of the oil/water phases takes place, meaning that a part of the oil phase reaches the top portion of said jacket from where it is removed. Thereby, that part of the oil phase does not reach the filter and a corresponding prolongation of the operating periods between filter cleaning operations is achieved.
With a proper filter material, the last-mentioned type of filter apparatus provides a very efficient cleaning. Not only droplets of oil or fatty liquids and particles, even of sub-micron size, are removed but also lipophillic pollutants dissolved in the aqueous phase can be removed. The apparatus has also other advantages, including high capacity and moderate space requirements.
Cleaning of the filter is necessitated either when the amount of pollutants in the effluent water exceeds a fixed value or when the pressure drop over the filter becomes too high. Said cleaning is made by back-flush using a hot fluid.
Often the amount of oil droplets and saturated vapour bubbles which settles on the outer filter surface is important as to the duration of the operational periods between each back-flush operation.
In spite of the fact that each back-flush operation only lasts a few minutes, it is desired to increase the length of each operational period by minimizing the amount of oil droplets, vapour bubbles, and particles penetrating into the filter surface. This is according to the invention achieved by applying a special measure to separate a proportion as large as possible of the oil phase from the contaminated water before the latter reaches the filter surface and also to loosen oil droplets and vapour bubbles from the filter surface to avoid their penetration into the filter material and blocking thereof.